Recent technical innovation in the field of optical disc is multiplication of recording layers for increase in capacity. Multilayered optical discs, however, suffer interlayer crosstalk during playback on an ordinary optical drive unit.
Crosstalk from a multilayered optical disc results from the detecting optical system in the optical pickup device as explained below with reference to FIG. 3, on the assumption that tracking error signals are detected by DPP (Differential Push-Pull) method. The DPP method consists of dividing a laser beam into one main beam and two sub-beams by means of a diffraction grating and allowing the three beams to irradiate an optical disc. Only the main beam 80 is shown in FIG. 3. For the sake of brevity, a dual-layered optical disc is denoted by 501 and information recording layers are denoted by 511 and 512. The main beam 80 through the objective lens 401 makes the minimum beam spot on the information recording layer 511 so as to read out information from it. On the information recording layer 511 are formed guide grooves for tracking as shown in FIG. 4. The main beam forms the optical spot 94 that irradiates the guide groove. At the same time, the sub-beam forms the optical spots 95 and 96 that irradiate positions away from the guide track by half a track pitch. Since the irradiating light is focused on the recording layer 511, its reflected ray returns to the objective lens 401 (shown in FIG. 3) along the same optical path as the incident ray. The reflected ray further passes through the detecting lens 402 (to become the beam 801) and enters the optical detector 51. The detecting lens 402 has astigmatism, and the optical detector 51 is placed at the position where the least circle of confusion occurs.
The optical detector has a shape as shown in FIG. 5 and the reflected ray from the optical disc has an incident pattern as shown in FIG. 5. The optical detector 541 (consisting of four sections divided by a cross) shown at the center is to detect the main beam. The main beam (in the form of optical spot 811) irradiates the detector 541. The two reflected rays from the sub-beams enter (forming the optical spots 812 and 813) separately the detectors 542 and 543, each of which is divided into two sections. Signals from the detector 541 (divided into four sections) are designated as A, B, C, and D, signals from the detector 542 (divided into two sections) are designated as E and F, and signals from the detector 543 (divided into two sections) are designated as G and H. Then, the tracking error signal (TR) is expressed as follows:TR=(A+B)−(C+D)−k{(E−F)}+(G−H)}where k is a constant to be determined by the ratio in intensity between the main beam and the sub-beams. Usually, the main beam is more than ten times as intensive as the sub-beams. Also, the focus error signal (AF) is expressed by AF=A+C−(B+D), and the data signal (RF) is expressed by RF=A+C+B+D. TR and AF are used to control the position irradiated with the laser beam.
The multilayered optical disc is designed such that, when it is irradiated with a laser beam, each layer reflects approximately the same amount of light to be detected by the optical detector. Consequently, the layer close to the objective lens has a larger transmittance than the layer away from the objective lens, so that the laser beam also irradiates the layer away from the objective lens. The problem that arises under this condition is that when the laser beam is focused on the layer 511 for information retrieval as shown in FIG. 3, a portion of the laser beam (designated as the beam 82) passes through the layer 511 and then it is reflected by the adjacent layer 512. The reflected beam 83, which is stray light, returns to the objective lens 401, enters the detection lens 402, becomes condensed in front of the optical detector 51, and finally enters the optical detector 51 while spreading along the optical beam 804. The optical beam 804 forms a wide optical spot 841 on the surface of the optical detector as shown in FIG. 5. The wide optical spot 841 covers the optical detectors 541, 542, and 543, and hence it interferes with the beams 811, 812, and 813. This interference depends on the phase variation of the optical spot 841 which occurs as the interlayer distance fluctuates.
Fluctuations in the intensity of RF signals (which are the total light quantity of the beam 811) deteriorate the jitter, thereby aggravating the error rate at the time of data reading. Moreover, interference between the beam 812 and the beam 813 causes the TR signals to fluctuate. This interference produces a strong effect because the sub-beams resulting from division by the diffraction grating have a low intensity nearly equal to the power density of the main beam reflected by the adjacent layer. This interference is also affected by the inclination and interlayer spacing of the optical disc. Thus, an optical disc with an uneven interlayer spacing causes the optical spot 812 or 813 to fluctuate in the distribution of light quantity as it rotates. This affects the differential portion (E−F)+(G−H) of the TR signal, resulting in unbalanced tracking signals. This in turn causes tracking errors. Likewise, the adjacent layer 512 closer than necessary to the layer 511 being read gives rise to the reflected light that causes troublesome interference.
One technology to reduce crosstalk is disclosed in Japanese Unexamined Patent Publication No. 2008-135097. According to this disclosure, the optical system for the pickup unit is provided with the reflection region limiting mirror 43 as shown in FIG. 7. A detailed description of this optical system is given below. There is shown the semiconductor laser 101 which emits a laser beam. The laser beam is converted into a circular collimated beam by the collimator lens 403 and triangular prism 102. The collimated beam is divided into three beams, one main beam and two sub-beams, by the diffraction grating 103. The main beam travels in the same direction as the incident beam, and the sub-beams travel in the directions inclined outward from the optical axis. The three beams pass through the polarizing beam splitter 104, change into circularly polarized light through the λ/4 plate 105, and focus on the multilayered disc 501 (being turned by a rotating mechanism) through the objective lens 404. Although the multilayered disc 501 shown here is a dual-layered disc, it may be replaced by any multilayered disc having three or more layers. The active layer (for reading) is indicated by 511, and the minimum spot of laser beam exists on the layer 511. The adjacent layer 512 also emits the reflected light 83 which is stray light that causes crosstalk.
The reflected light (including stray light) from the multilayered disc returns to the objective lens 404 and then passes through λ/4 plate 105 for conversion into the linearly polarized light in the direction perpendicular to the direction of the original polarized light. The linearly polarized light is reflected by the polarizing beam splitter 104 toward the λ/4 plate 106 for conversion into the circularly polarized light, which is subsequently condensed by the reflecting light condenser lens 405 and then reflected by the reflector 43 which is placed at the position of the minimum spot of the reflected light coming from the active layer 511. The reflector 43 has a shape as shown in FIG. 8. Reference numeral 831 denotes the minimum spot of the main beam, and Reference numerals 832 and 833 denote the minimum spots of the sub-beams. Their respective minimum spots are reflected by the restricted reflection regions 431, 432, and 433. Each of the reflection regions has a peripheral region with a low reflectivity. The beam which has been reflected by the active layer and then reflected by the reflector 43 returns to the reflecting light condenser lens 405 and then passes through the λ/4 plate 106 for conversion into the linearly polarized light orthogonal to the direction of polarization of the incident light. The resulting polarized beam passes through the beam splitter 104 and further passes through the condenser lens 406 having astigmatism and finally reaches the optical detector 52 placed at the position of the least circle of confusion. The optical detector 52 has sensitive parts as shown in FIG. 5. Signals from the optical detector 52 are processed by the signal processing circuit 53, so that there are generated AF signals and TR signals to control the position of the optical spot and RF signals as data signals.